IP routing depends on a well-structured hierarchy. Routers interconnect networks and send data from one network to another. Networks can interconnect with other networks and contain subnetworks. Private networks are commonly connected to the Internet through one or more routers (which may also be known as a gateway) so that devices on the private network can communicate with nodes on the Internet.
When a block of information or packet is sent from the Internet, routers will look only at the first few bits of an IP address and forward the packet to the current network delineated by a router. Each IP address has a four octet format. Typically, humans communicate IP addresses in a dotted decimal format, with each octet written as a decimal integer from other octets by decimal points. Routers further out look at the next few decimal integers, sending the packet to a subnet, which may be a local area network (LAN). The LAN router will look at the final decimal integer of the IP address and send the packet to a specific machine. Within a LAN, or a subnet, data is delivered using a physical MAC address assigned to each network interface card (NIC). This is a fixed address tied to an Ethernet card. The Address Resolution Protocol (ARP) maps the IP address to the MAC address. The router follows the APP and sends out a broadcast message asking for the device associated with the particular IP address to respond with its MAC address. Outside of the subnet, delivery based on the MAC address is not possible since there is no logical relationship to their number and the location on the network they reside. Data is routed to a next higher subnet. If the destination is there, the MAC address is resolved and the data is delivered. Therefore, outside of the subnet, the IP address is used for routing. Inside the subnet, the MAC address is used for delivery.
For a situation where there are mobile devices, such as PDAs, mobile telephones, and laptop computers, attached to an access point, the mobile devices' MAC address is associated with an IP address from within the subnet router's IP address space. If a user desires to take the mobile device that is attached to one access point and travel with it so that is attaches to a different access point within the same subnet, all that is necessary is for the new access point to respond to the MAC address of the mobile device that has just entered, and the previous access point to cease to respond to that MAC address.
If, however, the mobile device moves from one subnet to another, the mobile device cannot communicate over the new (remote) subnet. First, the mobile device is usually configured to send messages through a specified router at its original (home) subnet. Because it is no longer present at the home subnet and the specified router cannot be immediately located, communications from the mobile device will not be sent by the remote subnet. In addition, communications to the mobile device will be routed to the mobile device's home subnet. Because the router there will not know where to forward the packet, the communications will be lost. The result is that either the remote subnet must reconfigure its list of available IP addresses to accommodate the visiting mobile device, which is a burdensome and costly process, or the mobile device must acquire a new IP address each time it logs onto a new network or subnet. As such, the mobile device will be required to re-register with the LAN and may be required to re-enter a personal identification number (PIN) or some other password when connecting to a new subnet. Thus, seamless hand-offs can only be done within a subnet and not across different subnets.
FIG. 1 illustrates the currently available approach to manage roaming between subnets for a mobile telephone or other mobile device. As illustrated, the Internet 11 allows mobile devices on a home subnet 3 to communicate with mobile devices on a remote subnet 5 through a conventional wired router 12. In this specific example, a mobile device 20 having an IP address is normally connected to home subnet 3. In other words, the subnet 3 is the home subnet for mobile device 20.
For a situation where mobile device 20 migrates from its home subnet 3 to the remote subnet 5, either the mobile device 20 or the subnet 5 must undergo some transformation. As mentioned before, either the mobile device 20 must acquire a new IP address or the remote subnet 5 must reset its list of available IP addresses to accommodate the mobile device 20.
What is needed is a system and method for maintaining a permanent IP address when roaming across different subnets of a wireless network without requiring the remote subnet to undergo some transformation.